WO2015114278A1

WO2015114278A1 - Method and system for visualising infrared electromagnetic radiation emitted by a source

Info

Publication number: WO2015114278A1
Application number: PCT/FR2015/050233
Authority: WO
Inventors: Eric Freysz; Benoit PHILIPPEAU
Original assignee: Universite de Bordeaux; Centre National De La Recherche Scientifique
Priority date: 2014-02-03
Filing date: 2015-02-02
Publication date: 2015-08-06
Also published as: FR3017209A1; US10260955B2; EP3102917A1; FR3017209B1; US20170010161A1

Abstract

The present invention relates to a method and system for visualising infrared radiation emitted by a source. According to the invention, the visualising system comprises a substrate, at least one solid temperature-sensitive layer containing thermochromic pigments having at least one transition temperature T_H associated with a visible colour change, and at least one intermediate layer placed between the substrate and the solid layer containing thermochromic pigments; the substrate, said at least one intermediate layer and the solid temperature-sensitive layer containing thermochromic pigments forming a multilayer suitable for absorbing infrared electromagnetic radiation and for inducing a local temperature increase dT; and said at least one intermediate layer being suitable for transferring, by thermal conduction, said local temperature increase dT to a region of the solid temperature-sensitive layer, so as to induce a local change in the visible colour of the solid temperature-sensitive layer of thermochromic pigments.

Description

Method and system for viewing infrared electromagnetic radiation emitted by a source

Technical area

The present invention relates to a method and a system for viewing infrared electromagnetic radiation.

In the present document, electromagnetic radiation of infrared frequency is understood to mean electromagnetic radiation whose wavelength is greater than 1 micron and less than 20 microns.

State of the art

In an optical assembly, when the spectrum of the electromagnetic radiation is located outside the visible spectrum, especially in the infrared, a display card is generally used to detect, position or observe the spatial distribution of this electromagnetic radiation.

There are different visualization systems depending on the spectral range of the electromagnetic radiation considered. In particular, we know easy to use visualization cards and inexpensive.

The different visualization systems are based on different types of interactions between a material and an infrared electromagnetic radiation, these different interactions modifying the optical properties or the physicochemical properties of the material under consideration.

A first type of interaction is based on the use of nonlinear optical materials that convert non-linear two- or three-photon interactions into a beam in the infrared range into a visible range. However, these non-linear optical systems are extremely complex, and they are limited to coherent laser beams of high intensity and a spectral range limited to a single wavelength for a particular material.

Another type of interaction relies on the use of phosphorescent or fluorescent materials that have the property of absorbing infrared radiation and re-emitting radiation in the visible range. Thus, the document US5772916 describes a screen comprising a phosphorescent powder deposited on a substrate for detecting or locating a source of infrared radiation. Similarly, US6340820 discloses a display card comprising an active layer of phosphorescent components deposited on a substrate, for detecting and measuring the profile of a beam in the near-infrared range of 0.8 to 1.7 microns. The phosphorescent display cards are sufficiently sensitive to allow viewing of infrared radiation emitted by a light emitting diode. However, the sensitivity of phosphorescent visualization maps in the near infrared depends on the ambient illumination. On the other hand, the use of phosphorescent visualization maps meets limits for infrared radiation wavelength greater than two microns or high power density. High power density radiation can disable the sensitive area of the display board. In this case, an ultraviolet (UV) lamp is used to reactivate the phosphorescent compound sensitive surface and thus to visualize the trace of the infrared beam. This method is obviously much less convenient than a direct visualization. In any case, phosphorescent display cards are ineffective to visualize far infrared radiation, from 2 to 20 microns.

For example, GB2235060 discloses a device and method for viewing visible and near infrared radiation limited in a wavelength range of 600 nm to 1500 nm, which has a spectral bandwidth limited to a few hundred nanometers .

There are other infrared beam viewing systems based on interaction with liquid crystals or electronic systems. However, the latter systems are complex to use and generally quite expensive.

The patent document WO2014016525 describes a device and a method for viewing terahertz electromagnetic radiation which uses spin-transition materials.

There is a need for an inexpensive, easy-to-implement infrared electromagnetic beam viewing method and system having a threshold of sensitivity to infrared radiation exposure over a wide spectral range, preferably from 1 to 20 microns , for a power density ranging from 1 mW / cm ² to a few hundred W / cm ² , while being insensitive to variations in illumination or ambient temperature. There is also a need for a method and system for visualizing the spatial distribution of the power density of an infrared beam, sensitive over a wide spectral range.

The present invention aims to overcome the disadvantages of prior infrared radiation viewing systems and more particularly proposes a system for viewing the infrared electromagnetic radiation emitted by a source.

According to the invention, the display system comprises a substrate, at least one heat-sensitive solid layer comprising thermochromic pigments dispersed in a solid matrix, the thermochromic pigments having at least one transition temperature (T _H, T _B ), preferably greater at the temperature of use of the system, said at least one transition temperature (T _H, T _B ) being associated with an apparent color change of the thermochromic pigments in the visible, and at least one intermediate layer disposed between the substrate and the thermosensitive solid layer comprising thermochromic pigments, said at least one intermediate layer being thermally conductive; the substrate, said at least one intermediate layer and the thermosensitive solid layer together form a stack adapted to absorb infrared electromagnetic radiation and to induce a local increase in temperature dT; and said at least one intermediate layer is adapted to thermally transfer said local temperature increase dT to a region of the thermosensitive solid layer so as to induce a local change in the apparent color of the thermosensitive solid layer comprising thermochromic pigments.

Advantageously, the infrared radiation has a power density / within a range of 1 mW / cm ² to a few hundred W.cm ^-2 .

The visualization system is easy to use, inexpensive to manufacture and sensitive to a very wide range of wavelengths in the infrared. Advantageously, the display system of the invention is sensitive in a wavelength range which extends over a spectral bandwidth of several microns within a wavelength range of 1 micron to 20 microns. In an exemplary embodiment, the display system is sensitive in a wavelength range extending from at least 1.5 microns to 5 microns. In other words, this display system has a spectral bandwidth ranging from at least 1.5 microns to 5 microns. In another exemplary embodiment, the display system is sensitive in a wavelength range extending from at least 6 microns to 20 microns. In other words, this display system has a spectral bandwidth ranging from at least 6 microns to 20 microns.

The display system of the invention has an extended sensitivity range in terms of power density of incident infrared radiation and wavelength range.

Unlike systems in which the switching of the thermochromic pigments is photoinduced switching, the switching of the pigments is here thermally induced by changing the temperature of the system on which the pigments are deposited. This system does not require photocommutation properties or absorption of thermochromic pigments in the infrared range. The thermochromic pigments are essentially sensitive to the temperature increase of the matrix in which they are dispersed.

According to one aspect of the invention, said stack has an absorption coefficient a in a range of infrared wavelength, preferably between 1 and 20 microns, a density r and a heat capacity Cp defining a ratio: a

greater than a threshold value over said range of infrared wavelength, said ratio being adapted to induce by local absorption of infrared radiation an incident having a wavelength in said range of infrared wavelength and a power density /, a local temperature increase dT in said stack such that: rC _p

when the incident infrared radiation has a power density / within a range, preferably from 1 mW.cm ^-2 to 100 W.cm ^-2 .

Preferably, the material of the substrate is chosen from paper, cardboard, plastic, a metal, such as aluminum, copper, or a metal alloy, or a ceramic, for example alumina (Al ₂ O 3). ₃ ).

According to one aspect of the invention, said at least one intermediate layer comprises a contrast index layer in the visible formed of a material having a low absorption coefficient in the visible spectral range, said contrast layer of wherein the index is preferably a titanium dioxide (TiO ₂ ) loaded layer.

According to another aspect of the invention, said at least one intermediate layer comprises an infrared absorbing layer, said infrared absorbing layer having an absorption coefficient greater than 20% in at least a part of the length range. wavelength of between 1 and 20 microns, said absorbing layer in the infrared being preferably charged in carbon black.

Particularly advantageously, the display system comprises a first zone adapted to absorb a first infrared radiation having a power density in a first power density range, preferably between 5 W / cm ² and about 100 W / cm. ² , the display system being adapted to transform, by thermal conduction, an increase in temperature induced by absorption of the first infrared radiation into a local temperature increase dT1 greater than a thermochromic switching threshold in a first region of the thermosensitive solid layer of thermochromic pigments, so as to induce switching of the thermochromic pigment layer which produces a change in the apparent color of the thermochromic pigment layer when the first infrared radiation has a power density in the first power density range. The display system comprises a second zone adapted to absorb a second infrared radiation having a power density in a second power density range, preferably between 1 mW / cm ² and 5 W / cm ² , and the visualization is adapted to transform, by thermal conduction, an increase in temperature induced by absorption of the second infrared radiation in the stack into a local increase in temperature dT2 greater than a thermochromic switching threshold on a second region of the at least one solid layer thermosensitive thermochromic pigments, so as to induce a switching of the at least one layer of thermochromic pigments which produces another change in the apparent color of the at least one thermosensitive solid layer of thermochromic pigments when the second infrared radiation has a power density in the second power density range.

According to a preferred embodiment, the thermosensitive solid layer comprises at least one thermochromic pigment, spin-transition or otherwise, excluding liquid crystals.

According to another preferred embodiment, the thermosensitive solid layer comprises at least one thermochromic pigment chosen from leuco dyes.

In one variant, the thermosensitive solid layer comprises a mixture of at least two thermochromic pigments having different temperature transition thresholds.

According to one aspect of an embodiment, the thermochromic pigment layer comprises nanoparticles and / or microparticles of thermochromic pigments in an absorbent polymer matrix, preferably in the infrared range between 1 and 20 microns.

According to a particular and advantageous aspect of the invention, the substrate has a first face and a second face opposite to the first face, the second face of the substrate comprising at least one exposure zone having a bare surface adapted to receive said electromagnetic radiation incident infrared for transmission operation, and the first face being adapted to receive said incident infrared electromagnetic radiation for reflection operation.

The invention also relates to a method for viewing infrared electromagnetic radiation emitted by a source, comprising exposure to infrared radiation having a power density greater than or equal to a power density threshold, of at least one zone of a display system according to one of the embodiments, so as to induce by absorption in the stack and then by thermal conduction a local increase in the temperature in the thermochromic pigment layer greater than a transition threshold T _H , adapted to inducing an apparent color change of a region of the thermosensitive solid layer of thermochromic pigments.

The invention will find a particularly advantageous application in the detection and / or visualization maps of the spatial distribution of the power density of infrared electromagnetic radiation.

The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.

This description given by way of nonlimiting example will better understand how the invention can be made with reference to the accompanying drawings in which: FIGS. 1 and 4 show different manufacturing steps of an infrared radiation display system according to a first embodiment of the invention;

FIG. 5 illustrates an example of an infrared radiation display system, seen from the side of the face carrying the thermochromic pigment layer;

FIG. 6 illustrates the example of an infrared radiation display system of FIG. 5, seen from the opposite side to the thermochromic pigment layer;

FIGS. 7-10 illustrate various manufacturing steps of a display system according to a second embodiment of the invention;

FIGS. 11 and 12 illustrate another example of a two-zone display system with different sensitivity in terms of wavelength and / or in power density according to the second embodiment of the invention;

FIG. 13 illustrates different colors associated with different temperature ranges for a mixture of thermochromic pigments;

FIG. 14 illustrates an example of the visualization of the spatial distribution of the power density of infrared electromagnetic radiation from a mixture of thermochromic pigments;

FIGS. 15A-15B illustrate an exemplary embodiment of an infrared display card;

- Figures 16A-16D illustrates another embodiment of an infrared viewing card operating in reflection and transmission.

detailed description

The invention is based on the interaction between an infrared electromagnetic radiation, and a display system comprising a thermosensitive solid layer, the thermosensitive layer comprising a thermochromic material, or thermochromic pigment.

Thermochromic pigments have the ability to change their apparent color in the visible according to a change of temperature, heating or cooling. This color change may be related to a change in the shape of a molecule, for example in the case of a leuco dye (or leuco dye), to a phase change. Thus, at 68 ° C., the vanadium dioxide V0 ₂ has a phase transition, of a semiconductor state, which has a distorted monoclinic structure, in a metallic state, which has a rutile-like quadratic structure. Similarly, the BiV0 ₄ semiconductor, charge transfer material which is yellow at room temperature, has a red appearance at 300 ° C. Finally, this color change can be linked to a spin transition, in the case of a spin-transition material, as described for example in FR2894581 and FR2963015. A thermochromic pigment thus passes from a first color, at low temperature, to a second color, when the temperature increases above a temperature transition threshold T _H. For some materials, this transition is irreversible, for other materials it can be reversible. In the case where this transition is reversible, when the temperature falls below a certain temperature threshold T _B , a reverse color change is observed. Depending on the physicochemical nature of the thermochromic material, the temperature transition range for switching from one state to another state may range from a few degrees to several tens of degrees. In addition, many thermochromic materials have a thermal hysteresis loop.

Thermochromic materials can also exhibit photocommutation effect under infrared radiation of precise wavelength.

In particular, it is known from FR2963015, thermochromic compounds, spin transition type, in the form of powder or inserted in a polymer matrix, which allow to visualize a laser beam at a wavelength of 10.6 microns having a power density of between 1 mW / cm ² and 1 W / cm ² .

However, the thermochromic materials do not exhibit a photocommutation effect over a broad infrared spectral band, extending for example from 1 to 20 microns. On the other hand, spin-transition materials have low sensitivity.

In the field of thermochromic compounds, leuco-dyes offer a sharp color change in an interval of about 1 .5 ° C around the transition temperature T _H which can not currently be achieved with the materials to conventional spin transition.

The leuco-dyes are therefore very reactive to the change of temperature and thus allow to reach a high sensitivity.

More generally, the direct photocommutation of a particular material, for example with a spin transition, during the exposure of this spin-transition material to an infrared beam, has a spectrally limited sensitivity in the infrared, and It is not possible to visualize an infrared beam over a wide spectral range, nor to visualize the spatial profile of the power density of an infrared beam.

A thermochromic material taken alone is therefore not suitable for viewing an infrared beam having any wavelength in the range between 1 and 20 microns.

One observation forming part of the present invention is that a device of the prior art based on a phosphorescent material or on a particular thermochromic material can be adapted for the detection of an infrared beam in a relatively narrow spectral range and in a range. of reduced power density, but not suitable for detecting radiation having a wavelength in a wide infrared spectral range, and does not allow to visualize the spatial profile of the power density of an infrared beam.

There is provided a method and a system for detecting and / or visualizing the spatial distribution of the power density of infrared radiation based on the use of a system comprising a substrate 1, a solid layer 4 comprising particles of thermochromic pigment dispersed in a controlled amount in a matrix, and at least one intermediate layer (2, 3) disposed between the substrate and the solid layer 4.

The solid layer 4 comprising a thermochromic pigment is said to be thermosensitive, that is to say that it is sensitive to an increase and / or a drop in temperature with respect to at least one transition threshold T _H. However, a thermochromic layer is not necessarily a thermally conductive layer: the matrix in which the thermochromic pigments are inserted may be a thermally insulating matrix.

The operating principle of the visualization system is based on a local heating of the system following exposure to infrared electromagnetic radiation.

More specifically, the infrared electromagnetic radiation is absorbed by the display system and converted into a local temperature increase, which induces a color change, preferably reversible, of the thermochromic pigment used. The thermochromic pigment thus changes color indirectly, under irradiation of infrared radiation, when the temperature of the system increases locally above a transition threshold T _H of the thermochromic pigment. When the radiation is interrupted, the infrared absorption in the display system and induced heating cease: the system returns to equilibrium at room temperature, and the thermochromic pigment returns to its original color.

Thus, there is provided a display system comprising a stack, the stack comprising a substrate, a thermosensitive solid layer of thermochromic pigments having at least one predefined transition temperature (T _H , T _B ), and at least one intermediate layer, Stacking the display system having a predefined absorption coefficient for infrared radiation over an entire wavelength range of 1 to 20 microns, so that the display system absorbs infrared radiation. On the other hand, the stack has a density r and a heat capacity Cp which produce a local temperature increase in the stack and make it possible to transfer this increase in temperature in a region of the thermochromic pigment layer, which induces a change. the apparent color of the thermosensitive layer when the infrared radiation has a power density in a range, preferably from 1 mW.cm ^-2 to 100 W.cm ^-2 . The substrate used in the display system can be of different types.

The substrate 1 is preferably in the form of a thin plate, from a few tenths of a millimeter to a few millimeters in thickness.

For example, the substrate 1 may be paper, cardboard, wood, plastic, ceramic, metal plate.

In a variant, the display system has a sensitivity threshold preferably in the power density range of between a few mW.cm ^-2 to about 10 W.cm ^-2 , and a substrate is chosen which has a low thermal conductivity. , preferably less than 1 Wm ^-1 .K ^-1 at room temperature, such as paper, wood, a plastic material, such as PVC, so as to limit the heat losses of the display system to the ambient air . Therefore, such a display system has a relatively long relaxation time, of the order of 5s to 10s. It is noted that the relaxation time, for the return of the device to the original color, is a function of the power density of the infrared radiation.

In another variant, the display system has a sensitivity threshold preferably in the power density range of between 10 W.cm ^-2 to about 100 W.cm ^-2 and a substrate is chosen which has a high thermal conductivity. , preferably greater than 50 Wm ^-1 .K ^-1 at room temperature, such as a metal plate or a ceramic plate, for example alumina. Such a display system is more resistant to a power density of high infrared radiation, and has a short relaxation time, of the order of 3s.

The detection principle of the display system of the invention is based on a color change of at least a portion of the thermochromic pigments dispersed in a solid matrix deposited in a layer on a substrate, the system comprising at least one other intermediate layer between the heat-sensitive layer and the substrate.

To determine the threshold of detection of electromagnetic radiation, it is necessary to take into account different physical parameters of the complete visualization system.

The relationship between the temperature change and the physical parameters of the visualization system is as follows: r.Cp

where dT is the change in temperature per unit of time, / the power density per unit area of incident electromagnetic radiation, has the absorption coefficient of the display system, r the system density and Cp the heat capacity of the system. The absorption coefficient a of the display system is generally a function of the wavelength. Thus, to reduce the sensitivity threshold and increase the induced temperature increase dT for a fixed energy density per unit area, it is preferable to select a system having a high absorption coefficient at a low density r and / or or a low heat capacity Cp.

Once the physical parameters of the selected system, choose a thermochromic pigment having a transition temperature above the use temperature, for example ± 5 ° C above the temperature of use.

It is observed that the thermochromic pigments are not used here to directly absorb the infrared radiation but as an indicator of a temperature change of the display system. The thermochromic pigments are selected essentially according to their transition temperature and their apparent color in the visible. The thermochromic pigments are advantageously dispersed in a liquid matrix, which is solidified after deposition on the substrate and the intermediate layer. The amount of dispersed pigments depends on the initial color and the desired color change. By way of example, the quantity of dispersed pigments is between 5% and 20% by weight of the total weight in an ink forming the thermosensitive solid layer loaded with thermochromic pigments.

According to one advantageous aspect, the intermediate layer or layers disposed between the substrate and the heat-sensitive layer make it possible, by reflection or transmission, to provide a visual contrast enabling the user to better visualize a color change.

In a complementary manner, the intermediate layer or layers can also absorb part of the infrared electromagnetic radiation.

FIGS. 1 to 4 represent various manufacturing steps of an infrared radiation display system according to a first embodiment of the invention, which is compatible with a high electromagnetic radiation power density, of the order of 10 W .cm ^"2 at about 100 W.cm ^{" 2} .

Figure 1 shows a substrate 1. Advantageously, the substrate 1 is a substrate having good mechanical strength at high temperatures and good thermal conductivity (greater than 50 Wm ^-1 .K ^-1 at room temperature). For example, the substrate 1 is formed of a metal plate or a ceramic plate, such as alumina. The thickness of the substrate is preferably between a few tenths of a millimeter and a few millimeters.

A layer 2 is deposited on a first face of the substrate, which covers the entire surface of the first face. Advantageously, layer 2 is an infrared absorbing layer, for example composed of carbon black, which is absorbent over a very wide spectral range from visible to infrared. For example, a varnish loaded with carbon black (UVISOFT range) from VFP Ink Technologies is used.

FIG. 2 represents the substrate 1 of FIG. 1, seen from the second face, which is opposite to the first face. In FIG. 2, a layer 12 is deposited which makes it possible to delimit the zone of exposure to infrared radiation in the transmission mode. The transmission mode exposure zone therefore has no deposit and is indicated by the bare surface 13 of the substrate 1, which is opposite to the thermosensitive zone 4.

In FIG. 3, represents the substrate 1 of FIG. 1, seen from the first face. A contrast layer of index 3, preferably in opaque white lacquer, which covers a portion of the absorbent layer 2 is deposited. In one exemplary embodiment, the index contrast layer 3 is a lacquer loaded with carbon dioxide. Titanium (TiO ₂ ) from VFP Ink Technologies (UVISOFT range).

In FIG. 4, a thermosensitive layer 4 has been deposited comprising particles of thermochromic pigments dispersed in a matrix. For example, the heat-sensitive layer 4 is formed by depositing a colorless varnish from the company VFP Ink Technologies (UVISOFT range) loaded with thermochromic pigments. The color of the varnish may for example be orange or purple in the initial state below the transition threshold T _H.

In a particular embodiment, the matrix of the heat-sensitive layer 4 is a polymerizable varnish under irradiation with ultraviolet radiation. The heat-sensitive layer 4 is polymerized in thermochromic pigments in the same way, by exposure to ultraviolet radiation. In a variant, the heat-sensitive layer is formed of a colorless solvent-based varnish, which is fastness by evaporation.

According to a particular method, the various layers 2, 12, 3 and 4 are deposited by screen printing, in one or more passes.

The thickness of the absorbent layer 2, the index contrast layer 3 and the heat-sensitive layer 4 is preferably between a few tenths of a micron and a few hundred microns.

FIG. 5 represents a visualization system obtained according to the method detailed with reference to FIGS. 1 to 4, seen from the face carrying the thermosensitive layer 4. It is printed, for example by screen printing, in white ink on the layer 2 indications of operation of the device, such as the wavelength range, here for example from 6 to 20 microns. In another example, the wavelength range of the display system ranges from 1.5 to 5 microns.

The display system 10 of FIG. 5 thus operates in reflection. Infrared radiation 20, with a power density of 10 to 50 W / cm ² , is incident on the thermosensitive layer 4. When infrared radiation 20 is incident on the layer 4 loaded with thermochromic pigments, the infrared radiation can be absorbed by the matrix of the thermosensitive layer and / or by the contrast layer of index 3 and / or by the absorbent layer 2 and / or by the substrate 1. The absorbed infrared radiation is transformed into a local increase in the temperature of the display system 10. This temperature increase induced by absorption of the infrared radiation in the stack is transferred by thermal conduction to the heat-sensitive layer 4. When the increase in temperature of the thermosensitive layer 4 is greater than the transition temperature T _H , it induces a transition of the thermochromic pigments, and therefore a local change of apparent color of the thermosensitive layer in the visible. An observer thus detects a color change on the face of the thermosensitive layer directly exposed to infrared radiation 20.

FIG. 6 represents the display system of FIG. 5, seen from the face opposite to that carrying the layer 4 of thermochromic pigments.

The display system 10 of FIG. 6 thus operates in transmission. An infrared beam 21, with a high power density, of 50 to 100 W / cm ² , is incident on the bare surface 13 of the substrate 1. The infrared radiation can be absorbed by the substrate 1 and / or by the different layers located on the opposite face, that is to say by the absorbent layer 2 and / or by the index contrast layer 3 and / or by the matrix of the heat-sensitive layer 4. The infrared radiation 21 thus absorbed induces a local increase in the temperature in the stack of the display system 10. This temperature increase induced in the stack is transferred by thermal conduction to the layer thermosensitive 4. When the increase in temperature of the thermosensitive layer 4 is greater than the transition temperature T _H of the thermochromic pigments, it induces a transition of the thermochromic pigments, and thus a visible local change of color of the thermosensitive layer in the visible . An observer thus detects a color change on the face opposite to the face exposed to infrared radiation 21.

In transmission, the layer of thermochromic pigments is not exposed directly to electromagnetic radiation. The mechanical strength of the substrate allows the system to withstand high power densities, for example of the order of 100 Wcm- ² .The use of the transmission visualization system makes it possible to avoid damaging the thermosensitive layer 4 due to direct exposure to infrared radiation 21 of high power density.

The power density sensitivity of the same display system is different depending on whether it is used in reflection mode (FIG. 5) or in transmission mode (FIG. 6). A single display system as shown in Figures 5 and 6 thus has two complementary operating ranges.

Such a display system 10 makes it possible to simply visualize and locate electromagnetic radiation of low and / or high power density. FIGS. 16A-16D illustrate the operation of a display system as described with reference to FIGS. 5 and 6. FIG. 16A shows an ambient temperature display card seen from the side of the thermosensitive layer 4 The heat-sensitive layer 4 is deposited in rectangular form and appears gray at room temperature. In FIG. 16B, the display card of FIG. 16A is represented at a temperature higher than the temperature of transition of the thermosensitive layer: the thermochromic pigments have switched, and the heat-sensitive layer appears uniformly green. In Figure 16C, there is shown the same display card at room temperature, exposed to infrared radiation at a wavelength of 10.6 microns, power density of 20 W / cm ² , in reflection mode. 16C, in the center of the thermosensitive layer, a lighter spot 40 is observed, which allows to visualize and locate the infrared radiation. In FIG. 16D, there is shown the same ambient temperature display card, exposed to infrared radiation at a wavelength of 10.6 microns, with a power density of 60 W / cm ² , in transmission mode. 16D, in the center of the thermosensitive layer, a lighter central spot 41 is observed, which makes it possible to visualize and locate the infrared radiation by transmission.

FIGS. 7 to 10 represent different manufacturing steps of an infrared radiation display system according to a second embodiment of the invention.

FIG. 7 represents a substrate 1, for example formed of a PVC blank card, for example laminated without a protective layer (or overlay). On a first face of the substrate 1, an absorbent layer 2 is deposited which covers the entire surface of the first face. Advantageously, layer 2 is composed of carbon black, which is absorbent over a very wide spectral range from visible to infrared. For example, a varnish loaded with carbon black from VFP Ink Technologies (UVISOFT range) is used. A layer of contrast 3 has been deposited on the layer 2 which covers the entire surface of the layer 2. Advantageously, the contrast layer 3 is an opaque white varnish. In an exemplary embodiment, the contrast layer 3 is a layer of a titanium dioxide (TiO ₂ ) -labelled varnish from VFP Ink Technologies (UVISOFT range).

In FIG. 8, a first thermosensitive layer 4 has been deposited comprising thermochromic pigment particles dispersed in a matrix on a region only of the surface of the contrast layer 3. For example, the first thermosensitive layer 4 is formed by depositing a colorless varnish (UVISOFT range) from VFP Ink Technologies company charged with thermochromic pigments.

In FIG. 9, another heat-sensitive layer 5 has been deposited comprising particles of thermochromic pigments dispersed in a matrix on another region of the surface of the contrast layer 3, distinct from the region of the first 4. For example, the heat-sensitive layer 5 is formed by depositing a colorless varnish (UVISOFT range) from the company VFP Ink Technologies loaded with thermochromic pigments, of another color, corresponding to another transition temperature (T _H ) and therefore at a sensitivity different from that of the first thermosensitive layer 4.

For example, in the state below their respective transition threshold, the color of the varnish of the first heat-sensitive layer 4 may for example be pink, and the color of the varnish of the other thermosensitive layer 5 purple or black.

In a particular embodiment, the varnish of the heat-sensitive layers 4 and 5 is a polymerizable varnish under irradiation with ultraviolet radiation. The heat-sensitive layer 4 is polymerized in thermochromic pigments in the same way, by exposure to ultraviolet radiation. Advantageously, the absorbent layer 2, the contrast layer 3 and the heat-sensitive layers 4 and 5 are deposited by screen printing in one or more passes.

FIG. 10 represents a visualization system obtained according to the method detailed with reference to FIGS. 7 to 9, seen from the face carrying the heat-sensitive layers 4 and 5. It is printed, for example by screen printing, in black ink on the contrast layer 3 of the operating indications of the device, such as the wavelength range, here for example from 1.5 to 5 microns.

FIG. 11 represents another example of a manufacturing method, analogous to the method illustrated with reference to FIGS. 7-8, and in which other heat-sensitive layers 14, 15 are deposited juxtaposed on a contrast layer 3, itself deposited on an absorbent layer 2 in the infrared, for example carbon black, covering the substrate 1.

FIG. 12 shows another example of a visualization system obtained according to the method detailed with reference to FIGS. 7, 8 and 11, seen from the face carrying the heat-sensitive layers 14 and 15 loaded with thermochromic pigments. Screening indications, such as the wavelength range, which ranges from 6 to 20 microns, have been printed, for example by screen printing, in black ink on the contrast layer 3.

The visualization systems illustrated in FIGS. 10 and 12 operate by reflection, the infrared radiation being incident on one of the layers of heat-sensitive pigments 4 or 5 for the display device 30 of FIG. 10, and respectively on one of the layers of heat-sensitive pigments. 14 or 15 for the display device 30 of FIG.

The use of a display system 30 may have two different sensitivity ranges depending on the power density of the electromagnetic radiation to be displayed.

On the other hand, two display systems 30 each having specific wavelength sensitivity ranges can be used. Particularly advantageously, a thermochromic pigment is chosen which reacts at ambient temperature, the transition temperature T _H of which is preferably above ambient temperature, which makes it possible to record a trace of the infrared radiation in a non-permanent manner. .

Alternatively, it is also possible to choose a material whose ambient temperature is located inside the hysteresis loop, which makes it possible to keep track of the infrared radiation for the time necessary for observation and then to erase it by heating. or cooling.

Preferably, a thermochromic pigment having the steepest possible transition range is used, i.e. the transition of all of the thermochromic pigment is spread over a narrow temperature transition range. Advantageously, leuco dyes are used which have the advantage of having a hysteresis loop in a range of only 1.5 to 2 ° C.

Advantageously, a thermochromic pigment is selected having a low temperature temperature transition temperature at high temperature, such that this transition temperature is in a range between +5 degrees and +20 degrees above ambient temperature. Thus, the thermochromic pigment is not very sensitive to ambient variations of a few degrees of ambient temperature and does not switch spontaneously.

The greater the difference between the operating temperature of the system and the transition temperature, the higher the power density required to obtain the switching of the thermochromic pigment and the higher the sensitivity threshold of the display card.

Among the thermochromic pigments, mention may be made of the spin-transition compounds described in patent documents FR2894581 and FR2917410 which are in the form of micrometric or nanometric particles. These compounds exhibit a variety of colors in each spin state depending on the composition, concentration, size and shape of the particles.

Advantageously, thermochromic pigments can be used those marketed by the companies Naxagoras Technology, Paint With Pearl, Solar Color Dust or even those in the form of thermochromic capsules from the French company Gem'lnnov. Thermochromic pigments have the advantage of being available on a wide range of apparent colors in the visible.

In addition, thermochromic pigments are available in a wide variety of colors and with a wide range of transition temperatures, making it possible to adapt the configuration of a display system to different applications.

More precisely, the value of the power density threshold that makes it possible to switch the thermosensitive layer depends in particular on the infrared absorption coefficient of the stack and the spectrum of the infrared radiation. In a third embodiment, a thermosensitive ink is used comprising a mixture of thermochromic pigments having different switching temperatures. For example, a first thermochromic pigment having a switching temperature T _H equal to 30 ° C. is mixed with a second thermochromic pigment having a switching temperature T _H equal to 42 ° C. and then a third thermochromic pigment having a switching temperature T. _{H is} equal to 60 ° C. This gives a heat-sensitive layer having four apparent colors indicating respectively four temperature domains: a first color below 30 ° C., a second color between 30 ° C. and 42 ° C., a third color between 45 ° C. ° C and 60 ° C and finally a fourth color above 60 ° C. FIG. 13 illustrates an example of a mixture of three thermochromic pigments associated with different apparent colors as a function of the temperature ranges. When the heat-sensitive layer of the pigment mixture is at a temperature below 30 ° C, its apparent color 45 is blue. Then, when the same heat-sensitive layer is at a temperature between 30 ° C and 42 ° C, its apparent color 46 is light blue. For a temperature between 45 ° C and 60 ° C, the apparent color 47 of the heat-sensitive layer becomes green. Finally, for a temperature above 60 ° C, the apparent color is white.

Fig. 14 illustrates the exposure of a display system according to the third embodiment to infrared radiation at a wavelength of 10.6 microns with a power density of 10 W / cm ² . The visualization system comprises a stack formed of a substrate, an intermediate layer and a thermosensitive layer. The thermosensitive layer 4 here comprises a mixture of different thermochromic pigments. In the center of the thermosensitive layer 4, there is a spot on which the four colors can be distinguished: the white color 48 in the center, where the power density of the radiation is the higher, the green color 47 in the form of a ring surrounding the center, the light blue color 46 surrounding the green ring 47, and the blue color 45 over the rest of the surface of the heat-sensitive layer. Each color is associated with a temperature range. The apparent color of the heat-sensitive layer of the pigment mixture thus makes it possible to determine the temperature range. After calibration, this color range can be associated, for a determined range of infrared wavelengths, with different ranges of power density of infrared radiation.

A thermosensitive layer comprising such a mixture of different thermochromic pigments thus makes it possible to visualize the spatial distribution of the power density of an electromagnetic radiation.

However, the thermal conductivity of the system can induce thermal diffusion in the system, detrimental to the spatial resolution of the color change.

In a variant, the mixture of thermochromic pigments in the form of pads is deposited on a low-absorbency support and having good conductivity thermal. This variant makes it possible to visualize the spatial distribution of intensity of the incident radiation.

Advantageously, the choice of pigments makes it possible to select different temperature ranges and different colors.

The display system according to one or the other embodiments of the invention provides a simple and effective response to the problem of visualization of radiation invisible to the naked eye.

The implementation of such a visualization system is extremely simple. This simplicity of use is particularly advantageous in comparison with existing electronic infrared radiation viewing systems.

An infrared radiation display device has thus been manufactured preferably operating between 1 and 20 microns and with a spectral width extending over several microns. For example, the spectral band of the display device ranges from 1.5 to 5 microns and / or from 6 to 20 microns.

The duration of the exposure required to display the infrared radiation depends on many parameters, including the absorption coefficient of the stack, the type of thermochromic pigments of the heat-sensitive layer. In practice, the response time can range from a few milliseconds to a few tens of seconds.

FIGS. 15A-15B show an infrared display card, at ambient temperature, respectively in the absence of exposure to infrared radiation, and following exposure to an infrared beam having a wavelength of 2 microns and a power density 500 mW / cm ² . In FIG. 15A, the heat-sensitive layer is of uniform color. In Figure 15B, the spot 42 lighter in the center of the heat-sensitive layer 4, corresponds to the area of exposure to infrared radiation.

The visualization system undergoes a local temperature increase induced by absorption of infrared radiation, which produces a transition of the thermochromic pigments associated with an apparent color change of the thermosensitive layer. Outside the thermal diffusion zone, the remainder of the surface of the display card has not undergone a temperature increase sufficient to induce a transition of the thermochromic pigments and to change color. The light spot slowly fades out if the infrared radiation is blocked and the card regains its uniform color.

The invention makes it possible to visualize and locate radiation in the infrared wavelength domain.

An example of a display device has been tested for a power density of the order of 500 mW / cm ² with a laser source continuously emitting at the following wavelengths: 2 microns, 3 microns, and 4 microns, with a exposure time of a few seconds in reflection mode. Non-polarized radiation issued by this source is used directly without special precautions. The same visualization card makes it possible to visualize the infrared radiation at the various wavelengths indicated above.

An example of a display device operating in reflection and transmission has been tested with a C0 ₂ laser source delivering electromagnetic radiation at the wavelength of 10.6 microns. For this application, a thermal conductive substrate is used, preferably a ceramic material, such as alumina (Al ₂ O ₃ ). In reflection mode, the visualization card makes it possible to visualize a C0 ₂ laser beam having a power density of 20 W / cm ² , with an exposure time of a few seconds. In transmission mode, the same visualization card makes it possible to visualize a C0 ₂ laser beam having a power density of 60 W / cm ² , with an exposure time of a few seconds, without deterioration of the visualization system. The apparent size of the spot on the display map depends on the power density and the exposure time to the laser beam. The higher the power density and / or the exposure time, the larger the spot size. The size of the spot is not representative only of the spatial distribution of the beam.

The invention takes advantage of the absorption of infrared radiation by a stack of different materials, and the thermal conduction of this stack to a thermosensitive layer, to allow visualization of infrared radiation, in a spectral range and in a range of density. power ranging from low power density to high power density.

The invention provides a simple and effective response to a problem of viewing infrared radiation in a frequency range invisible to the naked eye.

The invention makes it possible to locate infrared radiation from a laser source or the like. The invention also makes it possible to characterize the spatial distribution of a laser beam of infrared radiation. Therefore, the invention facilitates the adjustment of an infrared optical assembly. The invention makes it possible to considerably extend the spectral range of visualization over a wide spectral range in the infrared.

Advantageously, the usable thermochromic pigments have a wide range of visualization colors in the visible. The implementation of the device and the visualization method of the invention is inexpensive. In addition, the invention is insensitive to variations in ambient temperature or ambient lighting.

The use of different thermochromic pigments having different transition thresholds as a function of the power density of the infrared radiation makes it possible to obtain display devices having a different sensitivity. It is thus possible to adjust the sensitivity of the device and display method according to the power of the infrared radiation that is to be visualized.

Claims

A display system (10, 30) for infrared electromagnetic radiation emitted by a source, the display system (10, 30) comprising:

a substrate (1),

at least one thermosensitive solid layer (4) comprising thermochromic pigments dispersed in a solid matrix, the thermochromic pigments having at least a transition temperature O ^" H, T _b ), said at least one transition temperature (T _H , T _B ) being associated with an apparent color change of said thermochromic pigments, and

at least one intermediate layer (2, 3) disposed between the substrate and the thermosensitive solid layer (4) comprising thermochromic pigments, said at least one intermediate layer (2, 3) being thermally conductive,

in which the substrate (1), said at least one intermediate layer (2, 3) and the thermosensitive solid layer (4) together form a stack adapted to absorb infrared electromagnetic radiation and to induce a local temperature increase dT and in wherein said at least one intermediate layer (2, 3) is adapted to heat transfer said local temperature increase dT to a region of the thermosensitive solid layer (4) so as to induce a local change in the apparent color of the thermosensitive solid layer (4) of thermochromic pigments.

The display system (10, 30) according to claim 1, wherein said stack has an absorption coefficient a in an infrared wavelength range, a density r and a heat capacity C _p , defining a ratio: a greater than a threshold value over said range of infrared wavelength, said ratio being adapted to induce by local absorption incident infrared radiation having a wavelength in said range of infrared wavelength and a density of power /, a local temperature increase c / T in said stack such that: when the incident infrared radiation has a power density / within a range, preferably from 1 mW.cm ^-2 to 100 W.cm ^-2 .

3. Display system (10, 30) according to one of claims 1 to 2, wherein the material of the substrate (1) is selected from paper, cardboard, plastic, metal or ceramic, preferably alumina.

4. Display system according to one of claims 1 to 3, wherein said at least one intermediate layer (2, 3) comprises a contrast index layer (3) in the visible formed of a material having a coefficient of low absorption in the visible spectral range, said index contrast layer (3) preferably being a layer filled with titanium dioxide.

5. Viewing system according to one of claims 1 to 4, wherein said at least one intermediate layer (2, 3) comprises an absorbent layer (2) in the infrared, said absorbent layer (2) in the infrared having an absorption coefficient of greater than 20% in at least a portion of the wavelength range of 1 to 20 microns, said infrared absorbing layer (2) being preferably charged with carbon black.

6. Display system according to one of claims 1 to 5, the display system (10, 30) comprising a first zone adapted to absorb a first infrared radiation having a power density in a first range of power density, preferably between 5 W / cm ² and about 100 W / cm ² , and the display system being adapted to transform, by thermal conduction, an increase in temperature induced by absorption of the first infrared radiation into a local increase in temperature dT1 superior at a thermochromic switching threshold in a first region of the thermochromic pigment layer, so as to induce a thermochromic switching of the thermochromic pigment layer which produces a change in the apparent color of the heat-sensitive solid layer (4) of thermochromic pigments when the first infrared radiation has a power density included in the first power density range, and the display system comprising a second zone adapted to absorb a second infrared radiation having a power density in a second power density range, preferably between 1 mW / cm ² and 5 W / cm ² , and the visualization system being adapted to transform, by thermal conduction, a temperature increase induced by absorption of the second infrared radiation in the stack into a local temperature increase dT2 greater than a thermochromic switching threshold on a second region of the at least one thermosensitive solid layer (4) of thermochromic pigments, so as to induce a switching of the at least one layer of thermochromic pigments which produces another change in the apparent color of the at least one heat-sensitive solid layer (4) of thermochromic pigments when the second infrared radiation has a power density included in the second power density range.

7. Display system according to one of claims 1 to 6, wherein the thermosensitive solid layer (4) comprises at least one thermochromic pigment selected from leuco dyes.

8. Display system according to one of claims 1 to 7, wherein the thermosensitive solid layer (4) comprises a mixture of at least two thermochromic pigments having different temperature transition thresholds.

9. Display system according to one of claims 1 to 7, wherein the substrate (1) has a first face and a second face opposite to the first face, the second face of the substrate comprising at least one exposure zone having a bare surface (13) adapted to receive said incident infrared electromagnetic radiation for transmission operation, and the first face adapted to receive said incident infrared electromagnetic radiation for reflection operation.

A method of viewing infrared electromagnetic radiation emitted from a source, comprising:

the exposure to infrared radiation having a power density greater than or equal to a power density threshold, of at least one zone of a display system according to one of claims 1 to 9, so as to induce by absorption in the stack and then by thermal conduction a local increase in the temperature in the thermochromic pigment layer greater than a transition threshold T _H , adapted to induce an apparent color change of a region of the thermosensitive solid layer (4 ) of thermochromic pigments.